[Toybox] [PATCH] Move the empty regex workaround into xregcomp.

[enh]

No current caller except grep needs this, but consistency seems like a
good idea.

Also change the xregcomp error message to be a bit more human-readable,
rather than mention an implementation detail.
---
 lib/xwrap.c       | 13 ++++++++++---
 toys/posix/grep.c |  5 ++---
 2 files changed, 12 insertions(+), 6 deletions(-)
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