split message print fuctions (mpr_*), strerrno, and mscp_get/set_error
into print.c/h and strerrno.c/h.
ToDo: revise usages of priv_set_errv and pr_* functions.
MSCP_SSH_AUTH_PASSWORD passes a password, and MSCP_SSH_AUTH_PASSPHRASE
passes a passphrase for publickey auth. They enable avoiding interactive
password input. Test cases are also added.
MSCP_BUILD_VERSION (`git describe --tags --dirty --match "v*"`) is
passed through include/mscp_version.h.in and cmake.
When git is failed, use VERSION file instead (for building from
source tar balls that excludes .git).
instead of ssh_estab_queue (delay-based approach). MaxStartups in
sshd_config limits number of conccurent incoming ssh connections.
mscp_opts->max_startups adjusts this value.
This commit runs mscp_prepare() in a pthread. mscp copy threads
run aysnchronously with mscp_prepare(). So, when mscp_prepare()
has not finished yet (due to too many source files), we can start
to copy files.
