Header file reorganization and cleaning up the public API for pppd version 2.5.0...

This commit does several things, being a squash-and-merge of a series
of changes; squashed in order not to break bisection.

* Clean up pppd.h, moving declarations that should only be accessed by
  pppd code (not by users of pppd) to a new pppd-private.h.  Also,
  other parts of pppd.h were moved to multilink.h, chap.h, eap.h,
  eui64.h, and a new options.h.

* Provide an API for access to data that is needed by plugins (in no
  particular order):
  - ifname
  - ifunit
  - remote_name
  - remote_number
  - peer_authname
  - status (now called "code" internally)
  - phase
  - doing_multilink
  - multilink_master
  - idle_time_limit
  - link_connect_time
  - max_connect_time
  - link_stats
  - ipparam
  - hostname
  - got_sigterm
  - got_sigusr2
  - got_sighup
  - session_number
  - maxoctets
  - maxoctets_dir
  - debug
  - persist
  - devnam
  - modem
  - peer_authname
  - sync_serial

* Update the version number to 2.5.0.

* Detect availability of stddef.h and stdarg.h.

* Rename some headers:
  - pppcrypt.c/h to crypto_ms.c/h
  - ppp-crypto.c/h to crypto.c/h
  - ppp-crypto-priv.h to crypto-priv.h
  - chap-new.c/h to chap.c/h

* Remove chap-md5.h, crypto-priv.h, eap-tls.h, etc. from the list of
  header files to be installed.

* Provide typedefs for the hook functions.

* Provide a typedef for the "phase" variable.

* Provide a typedef for the link statistics array.

* Remove the option_t typedef.

* Rename the following functions by adding a "ppp_" prefix (with the
  intention that these are a "public" API for use by plugins):
  - option_error
  - add_options
  - int_option,
  - options_from_file
  - script_setenv
  - bad_ip_adrs,
  - netif_get/set_mtu (renamed to ppp_get/set_mtu)
  - get_time
  - timeout
  - untimeout
  - safe_fork
  - sys_close
  - set_session_number
  - update_link_stats (renamed to ppp_get_link_stats)
  - add_notifier (renamed to ppp_add_notify)
  - remove_notifier (renamed to ppp_del_notify)
  - generic_[dis]establish_ppp (to ppp_generic_[dis]establish)

* Rename ppp_devnam to ppp_devname.

* Rename ppp_available() to ppp_check_kernel_support().

* Use unsigned char instead of u_char, unsigned short instead of
  u_short, uint32_t instead of u_int32_t.

* Add const to some declarations

* Update comments

* Change the interface for notifiers to use an enum to identify which
  notifier is to be modified.

* Provide an API for getting the path to a file, with an enum to
  identify different types of file.

* Link plugins with the -DPLUGIN flag

[paulus@ozlabs.org - wrote commit message]

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>