uemis: replace C-strings by std::string and std::string_view

The string code of uemis-downloader.cpp was broken in more ways
than can be listed here. Notably, it brazenly refused to free any
memory allocated for the parameters buffer.

Using std::string and std::string_view should plug all those
memory holes. That made it necessary to do some major refactoring.

This was done blind and therefore will break.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/subsurface-string.h

@@ -59,16 +59,17 @@ extern double ascii_strtod(const char *str, const char **ptr);
 }
 
 #include <string>
+#include <string_view>
 #include <vector>
 
 // Sadly, starts_with only with C++20!
-inline bool starts_with(const std::string &s, const char *s2)
+inline bool starts_with(std::string_view s, const char *s2)
 {
 	return s.rfind(s2, 0) == 0;
 }
 
 // Sadly, std::string::contains only with C++23!
-inline bool contains(const std::string &s, char c)
+inline bool contains(std::string_view s, char c)
 {
 	return s.find(c) != std::string::npos;
 }