[LinuxVersion] Cache linux version to avoid uname() syscall

Also add error handling to uname() call.

emper/lib/LinuxVersion.cpp

@@ -34,10 +34,17 @@ static auto checked_strtol(const std::string& s) -> long {
 
 namespace emper::lib {
 
+std::string LinuxVersion::globalVersion;
+
 LinuxVersion::LinuxVersion() {
-	struct utsname buf;
-	uname(&buf);
-	version = std::string(buf.release);
+	if (globalVersion.empty()) {
+		struct utsname buf;
+		if (uname(&buf)) {
+			DIE_MSG_ERRNO("Failed to invoke uname()");
+		}
+		globalVersion = std::string(buf.release);
+	}
+	version = globalVersion;
 }
 
 // Returns 1 if s is smaller, -1 if this is smaller, 0 if equal

emper/lib/LinuxVersion.hpp

@@ -9,6 +9,7 @@ class TestLinuxVersion;
 namespace emper::lib {
 class LinuxVersion {
 	friend class ::TestLinuxVersion;
+	static std::string globalVersion;
 	std::string version;
 
 	auto compare(const std::string& s) -> int;