On Linux, reads /proc/self/fd and resolves each symlink to identify what
the fd points to. Returns a data.frame with one row per open fd; useful
for diagnosing the cryptic "file descriptor is too large for select()"
failure from parallelly::makeClusterPSOCK().
openFds()data.frame with columns fd (integer), target (character, with
any Linux (deleted) suffix preserved), and bucket (character holder
category: "socket", "pipe", "terra scratch",
"terra scratch (deleted)", "tif (other)", "vrt", "sqlite",
"qs/qs2", "anon_inode", "other file", "unknown").
Returns an empty data.frame on non-Linux systems or if /proc/self/fd is
unreadable.
openFdsReport() for a printable summary; fdSelectLimit().