Constify get_init_creds string input params

The get_init_creds functions take read-only string input parameters
for passwords and initial ticket services.  Make these const char *
parameters instead of just char * parameters, for caller convenience.

Reported by rra@stanford.edu.

https://github.com/krb5/krb5/commit/6b5bc54f09763e6fd6531a1b83759b1aa7415136
Commit By: ghudson
Revision: 25858
Changed Files:
U   trunk/src/include/k5-int.h
U   trunk/src/include/krb5/krb5.hin
U   trunk/src/lib/krb5/krb/get_in_tkt.c
U   trunk/src/lib/krb5/krb/gic_keytab.c
U   trunk/src/lib/krb5/krb/gic_pwd.c
U   trunk/src/lib/krb5/krb/val_renew.c