lxc_create: prepend pretty header to config file (v2)

Define a sha1sum_file() function in utils.c.  Use that in lxcapi_create
to write out the sha1sum of the template being used.  If libgnutls is
not found, then the template sha1sum simply won't be printed into the
container config.

This patch also trivially fixes some cases where SYSERROR is used after
a fclose (masking errno) and missing consts in mkdir_p.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>