Change type of *hdrc variables to int

size_t's should only be used for the size of object in bytes, not just
for any general size of something. in this situation, an unsigned int is
probably the most appropriate, however this can potentially cause bugs
down the line with mixed sign arithmetic, so we'll just use ints.

also, argc is an int ;)
Refactor calloc() NULL checks in init

Proactively check for NULL right after each calloc() instead of doing
three callocs without checks, and then checking all of them at once.
Rename local variable for custom headers

...to avoid confusion with req_t field of same name. Also, I don't think
it was even an accurate name in the first place.
Make all non-api functions static

They shouldn't be called outside of requests.c anyway, so ensure that by
making them static.