Tighten parsing of M0/M1/M117/PRUSA

Expect these to be always at the start of the command (which is now
stripped of the extra line number), reducing the number of corner cases
due to these not following the g-code spec.

Firmware/Marlin_main.cpp

@@ -4071,9 +4071,10 @@ void process_commands()
         - TMC_SET_STEP
         - TMC_SET_CHOP
     */
-    if (code_seen_P(PSTR("M117"))) //moved to highest priority place to be able to to print strings which includes "G", "PRUSA" and "^"
+    if (strncmp_P(CMDBUFFER_CURRENT_STRING, PSTR("M117"), 4) == 0) //moved to highest priority place to be able to to print strings which includes "G", "PRUSA" and "^"
     {
-        lcd_setstatus(strchr_pointer + 5);
+        const char *src = CMDBUFFER_CURRENT_STRING + 4;
+        lcd_setstatus(*src? src + 1: src);
         custom_message_type = CustomMsg::M117;
     }
 
@@ -4091,8 +4092,9 @@ void process_commands()
     - `string` - Must for M1 and optional for M0 message to display on the LCD
     */
 
-    else if (code_seen_P(PSTR("M0")) || code_seen_P(PSTR("M1 "))) {// M0 and M1 - (Un)conditional stop - Wait for user button press on LCD
-        const char *src = strchr_pointer + 2;
+    else if ((strncmp_P(CMDBUFFER_CURRENT_STRING, PSTR("M0"), 2) == 0)
+          || (strncmp_P(CMDBUFFER_CURRENT_STRING, PSTR("M1 "), 3) == 0)) { // M0 and M1 - (Un)conditional stop - Wait for user button press on LCD
+        const char *src = CMDBUFFER_CURRENT_STRING + 2;
         codenum = 0;
         bool hasP = false, hasS = false;
         if (code_seen('P')) {
@@ -4246,7 +4248,7 @@ void process_commands()
 	}
 #endif //BACKLASH_Y
 #endif //TMC2130
-  else if(code_seen_P(PSTR("PRUSA"))){ 
+  else if(strncmp_P(CMDBUFFER_CURRENT_STRING, PSTR("PRUSA"), 5) == 0) {
     /*!
     ---------------------------------------------------------------------------------
     ### PRUSA - Internal command set <a href="https://reprap.org/wiki/G-code#G98:_Activate_farm_mode">G98: Activate farm mode - Notes</a>
@@ -6559,15 +6561,7 @@ Sigma_Exit:
       break;
 
       
-      /*
-        M117 moved up to get the high priority
-
-    case 117: // M117 display message
-      starpos = (strchr(strchr_pointer + 5,'*'));
-      if(starpos!=NULL)
-        *(starpos)='\0';
-      lcd_setstatus(strchr_pointer + 5);
-      break;*/
+    /* M117 moved up to get higher priority */
 
 #ifdef M120_M121_ENABLED
     /*!