sh(1): add -l option

-l is required by LSB for login shell, all other shells: bash, zsh,
oksh, mksh, ... implements it.

with -l sh will act as a login shell and read the profile.

MFC After: 1 week
Obtained From: dash (3b7c8442bfe7c2fd0a6b0415df6ddf66a399fd55)
Reviewed by: kib, lme
Differential Revision: https://reviews.freebsd.org/D47681

(cherry picked from commit 65f5dd42f11cdcb7716e6cdd09fac2314c144c1f)