Improve the PEAP contribution by Rustam Kovhaev

Improve the PEAP contribution by Rustam Kovhaev

These changes adds to his contribution by

  * Adding options to perform CA/CRL checking and certificate validation
    consistent with what is already been done for EAP-TLS
  * Certificate validation is now in line with what is already been done
    for EAP-TLS. Users can now set "remotename" and "tls-verify-method" to
    control these.
  * Validation of certificate purpose and extended key usage is controlled
    by the option "tls-verify-key-usage".
  * Fixing up MPPE key generation to use the new API for handling MPPE keys
  * Man page is updated where appropriate for the new options.
  * Added unit-tests for the PEAP code in case of crypto or parameters would
    change in the future.
  * Added the peap feature to configure scripts. Users can now control the
    feature by specifying --enable-peap/--disable-peap.

To acheive feature parity with the EAP-TLS change, the EAP-TLS common code was
refactored into tls.c/.h such that it could be re-used in both instances.

Using PEAP/MSCHAPv2 is now supported in PPPD with this change.

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>