dvtm 0.13

Updated dvtm to 0.13. Had to do a little inreplacing to work around OSX selectively removing SIGWINCH/VLNEXT in signal.h/termios.h when -D_POSIX_C_SOURCE is defined. Added a simple test.
@mikemcquaid, he does want to suport unix-like environments but does not want to complicate the build process to cover every edge case. He recommended that we just edit the config.mk file to suit our needs, which is effectively what inreplace is doing here.