Add new error code for "file name too long"

Add new error code for "file name too long"

This new error code, named file_name_too_long, maps internally to the
errno ENAMETOOLONG to produce a proper error code rather than an
internal code under errcode_for_file_access().  This error code can be
reached with some SQL command patterns, like a snapshot file name.

Reported-by: Alexander Lakhin
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/Zo4ROR9mgy8bowMo@paquier.xyz