user: reject requests to change full name to something including a comma

Because real name is stored in the GECOS field of /etc/passwd, which is delimited
by commas that cannot be escaped, the user's full name must not contain a comma.

Fixes #83
1 job for master in 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1773190
build-fedora

00:00:58