exec patch for clear text items

[Marc Phillips]

Thought this might be useful for others.

This is a first run through on an exec patch.  I'm sure
it can be greatly improved.  I've only tested it thus far
on Linux, but I don't see why it wouldn't work on other unix
platforms.

Adds in an exec option for any text field.  This is useful 
for me as I have policy that states I can't have any passwords
in clear text on the file system.  I have an exec that calls
out to an encrypted database for password retrieval:

Just have to change your clear text entry to: "exec:<your callout>"
Just like in Apache for SSL passwords, but is good for any text field.

It also keeps the logger from logging the return value:

*** freeradius-server-2.1.10/src/main/conffile.c        2010-09-28 06:03:56.000000000 -0500
--- freeradius-server-2.1.10-exec/src/main/conffile.c   2011-01-27 09:26:07.000000000 -0600
***************
*** 921,927 ****
                        if (!value) return -1;
                }

!               cf_log_info(cs, "\t%s = \"%s\"", name, value ? value : "(null)");
                *q = value ? strdup(value) : NULL;
                break;

--- 921,951 ----
                        if (!value) return -1;
                }

!               /* Add Callout for cleartext values */
!               if (strncmp(value,"exec:",5) == 0) {
!                       char *token;
!                       FILE *fileptr;
!                       char exec_return[1024];
!                       memset (exec_return,0x00,sizeof(exec_return));
!                       token = strtok(value,":");
!                       token = strtok(NULL,":");
!                       fileptr = popen(token, "r");
!                       if (fileptr == NULL) {
!                               return -1;
!                       }
!                       while (fgets(exec_return, sizeof(exec_return)-1, fileptr) != NULL) {
!                       }
!                       /* Get rid of carrage return from command output */
!                       exec_return[strlen(exec_return)-1]='\0';
!                       if (exec_return == NULL) {
!                               return -1;
!                       }
!                       pclose(fileptr);
!                       value = strdup(&exec_return);
!                       cf_log_info(cs, "\t%s = \"return from (%s)\"", name, token ? token : "(null)");
!               } else {
!                       cf_log_info(cs, "\t%s = \"%s\"", name, value ? value : "(null)");
!               }
                *q = value ? strdup(value) : NULL;
                break;