Automatically generate a man page

[dforsi]

Add --help and --version for help2man using only long options so that -h and -v are free to use for more common options.

Marked draft because there are some things that I don't like:

• the idea would be to have the man page automatically updated only if the output from --help or the version number change but help2man always adds the current month and year, so there could be 12 updates per year
• the use of magic numbers in the switch statement that handles --help and --version
• yet another version number embedded in a string (use a #define instead?)
• the rule to build the man page is too cryptic (but I think it can be reused as is for rtlsdr-ft8d), it means that rtlsdr-wsprd.1 should be regenerated only if rtlsdr-wsprd.c is newer and it should be regenerated after rtlsdr-wsprd is built
• README.md should mention help2man

[Guenael]

@dforsi Many good ideas! I never used help2man, but let's do this. But I don't think we have to put an installation requirement on help2man, if we setup an additional (additional) step in the Makefile.