This implementation parses the $HOME/.netrc (or a file specified by the
user) through the `--netrc[=file]` or `-R[file]` options.

It uses a simple FSM in order to parse the file and find the credentials
corresponding to the FTP host, or uses the "default" entry otherwise,
if any, according to the specification found on [1]. Tokens not
applicable to axel usage were not considered.

[1] https://docs.oracle.com/cd/E19455-01/806-0633/6j9vn6q5f/index.html

Closes: axel-download-accelerator#116

Signed-off-by: David Polverari <david.polverari@gmail.com>

Missing:

- add negative version (--no-netrc)

- add struct for netrc use, and use a pointer to it instead of using netrc_filename

- removed netrc_free
- simplified netrc_mmap
- reworked parser

- simplified parser flow
- replaced memcpy with strlcpy

This reverts commit c2fc839.