pluto_crypto_allocchunk: Avoid wrapround in assert
pluto_crypt_handle_dead_child: Remove, dead code
humanize_number: Avoid variable format string

Also add check for snprintf result.
get_addr: Move docstring comment in front of the function
LSW_FDMASK: Avoid signed integer overflow

Shifting into the sign position is currently a GCC extension, but that
may change in the future (according to the GCC manual).
format_connection: Avoid using the snprintf return value
biglset_format: Do not rely on the return value of snprintf
alg_info_snprint: Do not rely on the return value of snprintf
quick_inI1_outR1_authtail: Do not rely on the snprintf result