For +=/-=, add . and / to convenience characters

For +=/-=, add . and / to convenience characters

sysrc(8) supports key+=value and key-=value, but can be told what the
delimiter is by being passed as char1 (e.g., "sysrc key+=",value" to use a
comma as the delimiter instead of space). For convenience, if the first char
is alpha-numeric, it is assumed you wanted whitespace as the delimiter.

However, if you naively (as I just did) execute:
	sysrc rc_conf_files+=/etc/rc.conf.other
the result is unexpected.

This commit makes `.' and `/' in-addition to alpha-numeric first-characters
to cause the default of whitespace to be used as the delimiter. This also
means that you can no longer use these as a delimiter.