Fix building with C99 compilers

[fweimer-rh]

C99 removed implicit function declarations. Include <sys/stat.h> for the umask function, and init_history_timeout_timer because it is used in main.c.

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC