bsd-user: style tweak: move extern to header file

extern char **environ has no standard home, so move the declaration from the .c
file to a handy .h file. Since this is a standard, old-school UNIX interface
dating from the 5th edition, it's not quite the same issue that the rule is
supposed to protect against, though.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>