Userland: Allow building SerenityOS with -funsigned-char

Some of the code assumed that chars were always signed while that is
not the case on ARM hosts.

Also, some of the code tried to use EOF (-1) in a way similar to what
fgetc() does, however instead of storing the characters in an int
variable a char was used.

While this seemed to work it also meant that character 0xFF would be
incorrectly seen as an end-of-file.

Careful reading of fgetc() reveals that fgetc() stores character
data in an int where valid characters are in the range of 0-255 and
the EOF value is explicitly outside of that range (usually -1).

Userland/Libraries/LibSQL/Lexer.cpp

@@ -121,7 +121,8 @@ void Lexer::consume()
     auto did_reach_eof = [this] {
         if (m_position != m_source.length())
             return false;
-        m_current_char = EOF;
+        m_eof = true;
+        m_current_char = '\0';
         ++m_line_column;
         ++m_position;
         return true;
@@ -325,7 +326,7 @@ bool Lexer::is_line_break() const
 
 bool Lexer::is_eof() const
 {
-    return m_current_char == EOF;
+    return m_eof;
 }
 
 }