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.
