Pass by value to trim functions (s3fs-fuse#2345)

These already force a copy so passing by value has the same
performance but is simpler.  But this allows the compiler to perform
copy elision on temporaries and the caller to explicitly std::move in
others.

src/string_util.cpp

@@ -105,15 +105,13 @@ std::string lower(std::string s)
     return s;
 }
 
-std::string trim_left(const std::string &s, const char *t /* = SPACES */)
+std::string trim_left(std::string d, const char *t /* = SPACES */)
 {
-    std::string d(s);
-    return d.erase(0, s.find_first_not_of(t));
+    return d.erase(0, d.find_first_not_of(t));
 }
 
-std::string trim_right(const std::string &s, const char *t /* = SPACES */)
+std::string trim_right(std::string d, const char *t /* = SPACES */)
 {
-    std::string d(s);
     std::string::size_type i(d.find_last_not_of(t));
     if(i == std::string::npos){
         return "";
@@ -122,9 +120,9 @@ std::string trim_right(const std::string &s, const char *t /* = SPACES */)
     }
 }
 
-std::string trim(const std::string &s, const char *t /* = SPACES */)
+std::string trim(std::string s, const char *t /* = SPACES */)
 {
-    return trim_left(trim_right(s, t), t);
+    return trim_left(trim_right(std::move(s), t), t);
 }
 
 //

src/string_util.h

@@ -75,9 +75,9 @@ off_t cvt_strtoofft(const char* str, int base);
 //
 // String Manipulation
 //
-std::string trim_left(const std::string &s, const char *t = SPACES);
-std::string trim_right(const std::string &s, const char *t = SPACES);
-std::string trim(const std::string &s, const char *t = SPACES);
+std::string trim_left(std::string s, const char *t = SPACES);
+std::string trim_right(std::string s, const char *t = SPACES);
+std::string trim(std::string s, const char *t = SPACES);
 std::string lower(std::string s);
 
 //