Support auto-configuration of both Kerberos V4 and

Support auto-configuration of both Kerberos V4 and
Kerberos V5 for movemail, including detection of V4 and V5 header
files and libraries.
Include <string.h> when STDC_HEADERS is defined, to get
declarations of string functions.
[KERBEROS5] (socket_connection): Support the current MIT Kerberos
V5 API rather than the old one.
[KERBEROS] (socket_connection): Change a constant name from
SOCKET_ERROR to POP_SOCKET_ERROR to avoid a namespace conflict
with a constant in a header file.