sh: implement persistent history storage
Implement persistent history storage:
the strategy is simple at start: loads the existing .sh_history file
at exit dump it.
The implementation respects the HISTFILE variable and its POSIX
definition: ~/.sh_history is used if HISTFILE is not set.
to avoid sh to create the history file, set HISTSIZE to 0 or HISTFILE to
en empty value
Reviewed by: jilles
Differential Revision: https://reviews.freebsd.org/D29493