cgfs: improve read_file and append_line

cgfs: improve read_file and append_line

getline() returns the length which can be passed to
append_line to avoid a strlen() call.

Additionally with the length already known memcpy() can be
used instead of strcpy(). A +1 to the length will include
the terminating null byte as it is included in getline(3)'s
output.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>