replace usleep() with nanosleep() in msleep(), add a missing default case in getkey()

[sakhmatd]

usleep() has been removed in POSIX 2008, C code that includes rlutil.h will not compile if _POSIX_C_SOURCE is set to 200809L or greater.

nanosleep() from time.h is a suggested alternative and compiles successfully whether the POSIX standard is set or not.

Also, a default case has been missing in getkey() in the switch case for processing ASCII escapes. Not having a default case also causes C code to not compile when universally appropriate warning flags are turned on with gcc.

I set the default case to return -1, but there might be a better alternative (if maybe a #define to -1 somewhere for code readability).