Use asprintf for string construction

Use asprintf for string construction

Rather than using something along the lines of

    strOut = malloc(strlen(strA) + strlen(strB) + strlen(strC) + 1);
    strcpy(strOut, strA);
    strcat(strOut, strB);
    strcat(strOut, strC);

use asprintf for string construction, so we can just write

    asprintf(&strOut, "%s%s%s", strA, strB, strC);

roken provides an implementation of asprintf for platforms which are
missing one.

Change-Id: Ieef9f4b65f72260c0d372cdf3865daab98733ad9
Reviewed-on: http://gerrit.openafs.org/7451
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>