res_pjsip: Add to list of valid characters for from_user.

Fixes a regression where some characters were unable to be used in
the from_user field of an endpoint. Additionally, the backtick was
removed from the list of valid characters, since it is not valid,
and it was replaced with a single quote, which is a valid character.

ASTERISK-27387

Change-Id: Id80c10a644508365c87b3182e99ea49da11b0281