libjob: support f58plain encoding

Problem: flux_job_id_encode() cannot encode a job id
using FLUID_STRING_F58_PLAIN.

Add support for "f58plain" encoding.

Author: garlick

src/common/libjob/id.c

@@ -88,6 +88,8 @@ int flux_job_id_encode (flux_jobid_t id,
         t = FLUID_STRING_MNEMONIC;
     else if (strcasecmp (type, "f58") == 0)
         t = FLUID_STRING_F58;
+    else if (strcasecmp (type, "f58plain") == 0)
+        t = FLUID_STRING_F58_PLAIN;
     else if (strcasecmp (type, "emoji") == 0)
         t = FLUID_STRING_EMOJI;
     else {

src/common/libjob/job.h

@@ -84,7 +84,7 @@ int flux_job_id_parse (const char *s, flux_jobid_t *id);
 /*  Encode a jobid into encoding "type", writing the result to buffer
  *   buf of size bufsz.
  *  Supported encoding types include:
- *   "dec", "hex", "kvs", "dothex", "words", or "f58".
+ *   "dec", "hex", "kvs", "dothex", "words", "f58", or "f58plain"
  *  Returns 0 on success, -1 on failure with errno set:
  *   EPROTO: Invalid encoding type
  *   EINVAL: Invalid other argument