PR432215 Add debuginfod functionality

PR432215 Add debuginfod functionality

debuginfod is an HTTP server for distributing ELF/DWARF debugging
information.  When a debuginfo file cannot be found locally, Valgrind
is able to query debuginfod servers for the file using its build-id.

readelf.c: Add debuginfod_find_debug_file(). Spawns a child process to
exec `debuginfod-find` in order to query servers for the debuginfo
file. Also add helper debuginfod_find_path().

pub_core_pathscan.h: Moved from priv_initimg_pathscan.h in order to use
VG_(find_executable)() in readelf.c.

docs: Add information regarding debuginfod to valgrind.1

memcheck/tests/linux: Add new test debuginfod-check.

tests/vg_regtest.in: Clear $DEBUGINFOD_URLS before running any tests.

https://bugs.kde.org/show_bug.cgi?id=432215