Adapt MSDOS build according to changes in revno 103623.

 src/termcap.c [MSDOS]: Include "msdos.h.
 (find_capability, tgetnum, tgetflag, tgetstr, tputs, tgetent):
 Constify `char *' arguments and their references according to
 prototypes in tparam.h.
 src/deps.mk (termcap.o): Depend on tparam.h and msdos.h.
 src/msdos.c (XMenuAddPane): 3rd argument is `const char *' now.
 Adapt all references accordingly.
 src/msdos.h (XMenuAddPane): 3rd argument is `const char *' now.