analyze: add new "pcrs" verb

It shows the PCRs we know about along with their name strings and
current values.

src/shared/tpm2-util.c

@@ -4046,3 +4046,4 @@ static const char* const pcr_index_table[_PCR_INDEX_MAX_DEFINED] = {
 };
 
 DEFINE_STRING_TABLE_LOOKUP_FROM_STRING_WITH_FALLBACK(pcr_index, int, TPM2_PCRS_MAX - 1);
+DEFINE_STRING_TABLE_LOOKUP_TO_STRING(pcr_index, int);

src/shared/tpm2-util.h

@@ -221,4 +221,5 @@ int tpm2_util_pbkdf2_hmac_sha256(const void *pass,
                     size_t saltlen,
                     uint8_t res[static SHA256_DIGEST_SIZE]);
 
-int pcr_index_from_string(const char *s);
+int pcr_index_from_string(const char *s) _pure_;
+const char *pcr_index_to_string(int pcr) _const_;