pressure-vessel: Fix regression regenerating ld.so.cache on Ubuntu and older Debian

• pv-adverb: Expand error message if we cannot regenerate ld.so cache

  We can make it easier to diagnose ldconfig failures by including the stderr from the failing ldconfig command in our error message.

• pv-adverb: If we cannot regenerate ld.so.cache, fall back

  Before !338 (merged) (T14481), we relied on the LD_LIBRARY_PATH as our way to find libraries from the graphics stack provider (host system or Flatpak runtime), both /overrides/lib/MULTIARCH and /overrides/lib/MULTIARCH/aliases. However, this was not robust against games that (incorrectly) reset the LD_LIBRARY_PATH in their startup scripts, such as Dead Cells, Evoland Legendary Edition and Shenzhen I/O.

  Since !338 (merged), we maintain three separate library search paths:

  1. The LD_LIBRARY_PATH that is used to run pv-adverb is the same as before. (This is getenv("LD_LIBRARY_PATH") in pv-adverb code.)
  2. Library directories that contain libraries named by their SONAME (/overrides/lib/MULTIARCH) are prepended to ld.so.conf, after which we invoke ldconfig to regenerate ld.so.cache. This means games that reset the LD_LIBRARY_PATH will effectively still search /overrides/lib/MULTIARCH, as a result of the usual logic that reads ld.so.cache. (This is global_ld_so_conf_entries in pv-adverb code.)
  3. Library directories that contain libraries not named by their SONAME (/overrides/lib/MULTIARCH/aliases) still need to be in the LD_LIBRARY_PATH, because ld.so.cache only lists libraries by their SONAMEs, so we still need to put these library directories in the LD_LIBRARY_PATH before invoking the actual game. (This is opt_set_ld_library_path in pv-adverb code.)

  This worked fine on systems like Debian 11 and Arch, but is unnecessarily fragile: if running ldconfig failed, there was no fallback, and the game would simply not launch correctly. In particular, ldconfig can fail to run on Ubuntu and old Debian systems (prior to Debian 9), as a result of a workaround involving /sbin/ldconfig being made a shell script wrapper around /sbin/ldconfig.real.

  We can mitigate this failure by falling back to the old (pre-!338) logic if ldconfig fails, with a warning. Games like Dead Cells will still not launch successfully in this mode, but better-behaved games (including most native Linux titles and all Proton titles) will survive.

  A subtlety here is that when we fall back, we need to find both /overrides/lib/MULTIARCH and /overrides/lib/MULTIARCH/aliases in the LD_LIBRARY_PATH; so we discard search paths 2 and 3, and keep using search path 1 for the actual game, not just pv-adverb.

• pv-runtime: Try to create /sbin/ldconfig.real if using host glibc

  In Ubuntu and older versions of Debian, /sbin/ldconfig is a shell script that wraps /sbin/ldconfig.real. To make it possible to run /sbin/ldconfig, we have to map /sbin/ldconfig.real into the container too.

  Resolves: T29764

/cc @jpwhiting @vivek