libutil/fluid: add FLUID_STRING_F58_PLAIN

Problem: in non-UX sitations it may be safer or required
to encode FLUIDs using a prefix of f not ƒ, but there
is no convenient way to do that.

Add FLUID_STRING_F58_PLAIN encoding, which is the same as
FLUID_STRING_F58, except that the prefix is forced to f.

Update unit test.

Author: garlick

src/common/libutil/fluid.c

@@ -189,7 +189,7 @@ static inline int is_utf8_locale (void)
     return 0;
 }
 
-static int fluid_f58_encode (char *buf, int bufsz, fluid_t id)
+static int fluid_f58_encode (char *buf, int bufsz, fluid_t id, bool plain)
 {
     int count;
     const char *prefix = f58_prefix;
@@ -203,7 +203,7 @@ static int fluid_f58_encode (char *buf, int bufsz, fluid_t id)
 
 #if !ASSUME_BROKEN_LOCALE
     /* Use alternate "f" prefix if locale is not multibyte */
-    if (!is_utf8_locale())
+    if (!is_utf8_locale() || plain)
         prefix = f58_alt_prefix;
 #endif
 
@@ -332,7 +332,11 @@ int fluid_encode (char *buf, int bufsz, fluid_t fluid,
                 return -1;
             break;
         case FLUID_STRING_F58:
-            if (fluid_f58_encode (buf, bufsz, fluid) < 0)
+            if (fluid_f58_encode (buf, bufsz, fluid, false) < 0)
+                return -1;
+            break;
+        case FLUID_STRING_F58_PLAIN:
+            if (fluid_f58_encode (buf, bufsz, fluid, true) < 0)
                 return -1;
             break;
         case FLUID_STRING_EMOJI:

src/common/libutil/fluid.h

@@ -24,6 +24,7 @@ typedef enum {
     FLUID_STRING_MNEMONIC = 2,  // mnemonicode x-x-x--x-x-x
     FLUID_STRING_F58 = 3,       // FLUID base58 enc: ƒXXXX or fXXXX
     FLUID_STRING_EMOJI = 4,     // FLUID basemoji enc: 😪🏭🐭🍑👨
+    FLUID_STRING_F58_PLAIN = 5, // FLUID base58 enc: fXXXX
 } fluid_string_type_t;
 
 struct fluid_generator {

src/common/libutil/test/fluid.c

@@ -103,6 +103,14 @@ void test_f58 (void)
         "fluid_encode with FLUX_F58_FORCE_ASCII used ascii prefix");
     if (unsetenv ("FLUX_F58_FORCE_ASCII") < 0)
         BAIL_OUT ("Failed to unsetenv FLUX_F58_FORCE_ASCII");
+
+    ok (fluid_encode (buf,
+                      sizeof (buf),
+                      f58_tests->id,
+                      FLUID_STRING_F58_PLAIN) == 0,
+        "fluid_encode FLUX_STRING_F58_PLAIN works");
+    is (buf, f58_tests->f58_alt,
+        "fluid_encode FLUID_STRING_F58_PLAIN used ascii prefix");
 #endif
 
     ok (fluid_encode (buf, 1, 1, type) < 0 && errno == EOVERFLOW,