pgAdmin 4: Utility binary path can point at a shared storage directory

pgAdmin 4 runs pg_dump, pg_restore, pg_dumpall and psql from the binary path a user sets in their preferences. In server mode the binary path was refused only when it lay under the per-user storage directory (STORAGE_DIR), not under a SHARED_STORAGE directory, which ordinary users can upload files to. A user could therefore upload an executable to shared storage, point their binary path at it, and have pgAdmin run it as the operating-system account running pgAdmin. On POSIX, files created through pgAdmin are not executable, so this is exploitable where any file with an executable name can run, such as Windows server mode. The fix checks the binary path against every SHARED_STORAGE directory as well, after expanding $DIR, collapsing ".." and resolving symbolic links, compares directories in a way a different spelling on a case-insensitive filesystem cannot bypass, refuses a path containing a NUL, and enforces the check wherever binary paths are resolved, including for the PSQL tool. This issue affects pgAdmin 4: before 9.19.