Fix compiler warnings when char is unsigned

In several places, we check for a control character with something like

  char c;
  [...]
  if (c >= 0 and c <= 0x1F)
    [...]

When char is signed (e.g. amd64) this is fine, but when char is unsigned
by default (e.g. arm32 and arm64) this generates warnings about the
tautologous check that an unsigned value is non-negative.

Write as

  if ((unsigned char) c <= 0x1F)
    [...]

which is both correct and not suspicious under both conventions.

src/json.cc

@@ -20,7 +20,7 @@ String to_json(StringView str)
     for (auto it = str.begin(), end = str.end(); it != end; )
     {
         auto next = std::find_if(it, end, [](char c) {
-            return c == '\\' or c == '"' or (c >= 0 and c <= 0x1F);
+            return c == '\\' or c == '"' or (unsigned char) c <= 0x1F;
         });
 
         res += StringView{it, next};
@@ -28,7 +28,7 @@ String to_json(StringView str)
             break;
 
         char buf[7] = {'\\', *next, 0};
-        if (*next >= 0 and *next <= 0x1F)
+        if ((unsigned char) *next <= 0x1F)
             format_to(buf, "\\u{:04}", hex(*next));
 
         res += buf;

src/terminal_ui.cc

@@ -30,8 +30,8 @@ static String fix_atom_text(StringView str)
     auto pos = str.begin();
     for (auto it = str.begin(), end = str.end(); it != end; ++it)
     {
-        char c = *it;
-        if (c >= 0 and c <= 0x1F)
+        unsigned char c = *it;
+        if (c <= 0x1F)
         {
             res += StringView{pos, it};
             res += String{Codepoint{(uint32_t)(0x2400 + c)}};